FIG. 1 shows an example of an overall call processing procedure beginning from power turn on of a mobile station in a mobile communication system. When power is applied to the mobile station, necessary system information is read from memory and set in a RAM (or other storage means), synchronization with the system is performed by using a pilot channel and a sync channel, and a mobile station initialization state for receiving required system information is entered. In a message (e.g., a sync channel message) that is transmitted through the sync channel, system information, and frequency information related to a paging channel or a P-BCCH (Primary Broadcast Control CHannel) to allow a mobile station in idle state to decode the paging channel or the P-BCCH, and data rate or coding rate information are included. If the P-BCCH is employed, transmit diversity related information is additionally included.
The mobile station idle state refers to a state when, after the mobile station in initialization state received all system information and normal operations are complete, overhead messages have been received from a base station through the paging channel or the P-BCCH by using the information transmitted via the sync channel message. Some examples of these overhead messages are a system parameters message (SPM) that contains system related information, an extended system parameters message (ESPM), a general neighbor list message (GNLM) that contains information about neighbor cells or sectors, an extended neighbor list message (ENLM), a universal neighbor list message (UNLM), and the like.
The system access state refers to a state for allowing the mobile station to access the system, while the traffic state refers to when the mobile station is performing communications, i.e., a call is being performed. When a mobile station in system access state completes a particular operation, it returns to the idle state or traffic state. While in idle state, if a message transmitted through the paging channel or the P-BCCH is not received, the initialization procedure is started again. For a mobile station in traffic state, if a call ends while being performed, the initialization procedure is started again to re-establish sync.
The mobile station not only measures the pilot signal strength of a serving base station, but also obtains information, such as PILOT_PN and the like, with respect to neighbor cells, through neighbor messages such as GNLM, ENLM, UNLM, etc., and continuously monitors the pilot signal strengths of the neighbor cells. As a result of such monitoring, if the pilot signal strength of a neighbor cell increases above a certain threshold when compared to the pilot signal strength of the serving base station, the mobile station performs idle handoff, which refers to receiving a paging channel or P-BCCH from a base station having greater pilot signal strength.
Hereafter, the base station that the mobile station moves to upon performing idle handoff is called a target base station, while the current base station is called a source base station.
In the situation where a mobile station that had received a paging channel, moves to a target base station and must receive a P-BCCH, the related art suffers from problems because the mobile station does not receive from the source base station any information needed for decoding the P-BCCH. In more detail, as shown in FIGS. 2a and 2b, during idle state handoff, the idle state does not directly change over to idle state, but the mobile station moves to the target base station and after entering a system determination sub-state, the pilot channel and sync channel are received, information regarding the P-BCCH is received therefrom, and thereafter, idle state is entered. This causes the problem of creating delays.
Additionally, in the situation where a paging channel had been received from the source base station and a paging channel is to be received upon moving to the target base station, if the data rate changes upon moving to the target base station due to handover (e.g., changing from 4800 bps to 9600 bps), because the related art mobile station should go into the system determination sub-state and then enter the idle state, the problems of delay may occur. Similar problems occur when the related art mobile station that had received a P-BCCH from the source base station, needs to receive a paging channel after performing handover to the target base station.
Similarly, in the situation where a P-BCCH was received from the source base station and a P-BCCH is to be received from the target base station, if the data rate or coding rate or Walsh code changes at the target base station, the related art mobile station should go into the system determination sub-state and then enter the idle state, thus causing problems of delay.
If such delays occur during idle state handoff, if the related art mobile station had been receiving a broadcast service from the source base station while in idle state, these delays can greatly affect the broadcast service quality (QoS). In other words, as the related art mobile station in idle state receives a broadcast service through a forward supplement channel (F-SCH), the broadcast service cannot be received during the time consumed by the delay in entering the idle state by going through the system determination sub-state upon performing idle handoff.